1. Field of the Invention
The present invention relates to communications systems. More particularly, the present invention relates to wireless discrete multitone spread spectrum communications systems.
2. Description of Related Art
Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths and must make highly efficient use of the scarce bandwidth resource for providing quality service to a large population of users.
In the TDD PWAN system described in the copending US patent application by Siavash Alamouti, Doug Stolarz, and Joel Becker, entitled "VERTICAL ADAPTIVE ANTENNA ARRAY FOR A DISCRETE MULTITONE SPREAD SPECTRUM COMMUNICATIONS SYSTEM", Time Division Duplexing (TDD) is used by a base station and a remote unit for transmitting data and control information in both directions over the same multi-tone frequency channel. Transmission from a base station to a remote unit is called "forward transmission" and transmission from a remote unit to a base station is called "reverse transmission". The time between recurrent transmissions from either a remote unit or a base station is called the TDD period. In every TDD period, there are four consecutive transmission bursts in each direction. Data is transmitted during each burst using multiple tones. A base station and each remote unit must synchronize and conform to the TDD timing structure and both a base station and a remote unit must synchronize to a framing structure. All remote units and base stations are globally synchronized so that all remote units transmit at the same time and then all base stations transmit at the same time.
Further, since the TDD PWAN system uses a TDD format, the compensation measurements of the transmit and receive path circuitry are made during the respective idle times of the paths. The time domain duplexing of an airlink results in a 50% duty cycle for utilization of the transmit and receive circuits. Therefore, compensation measurements for the circuitry of a particular path are performed when an airlink does not require its use. Use of the transmit/receive duty cycle of the forward and reverse circuits for making transmit/receive compensation measurements frees system bandwidth and provides much greater measurement flexibility.
In the FDD PWAN system described in the copending US patent application by Alamouti, Michelson, et al., entitled "Method for Frequency Division Duplex Communications", Frequency Division Duplexing (FDD) is used by a base station and a remote unit for transmitting data and control information in both directions over different multi-tone frequency channels. The remote stations and the base station are frequency division duplexed (FDD) by transmitting their respective signals on different sets of discrete frequency tones in two different frequency bands separated by 80 MHZ. The FDD PWAN system needs to make compensation measurements of the transmit and receive path circuitry.
What is needed is a way to perform compensation measurements of the transmit and receive path circuitry of a dual FDD/TDD system which can be selectively operated in either the FDD mode or alternately in the TDD mode.